Optical networks in the future need, for example, to be formed over a conventional optical communication system, and to perform processing for branching, inserting, and switching control light of devices at a point separate from terminal devices such as optical repeater nodes. At this time, from the viewpoint of the efficient energy use, it is effective to minimize the number of conversions between optical signals and electric signals during propagation and processing of information.
In the currently used optical repeater nodes, signals are processed by using conversions between optical signals and electric signals as in the terminal devices. For example, control light transmitted to an optical repeater node is once converted into an electric signal, then electrically processed, and converted into an optical signal again. Therefore, the equipment structure is complicated, and great electric power is needed for compensating for loss caused by the conversions between optical signals and electric signals.
Meanwhile, for example, an optical network may monitor a variety of information in places thereof and transmit a monitor signal to other places thereof. For example, an optical signal processing device provided on a certain place of the optical network multiplexes a monitor signal of the place on which its own device is provided on carrier light propagating through an optical transmission line, and transmits the monitor signal to optical signal processing devices in other places.
In addition, conventionally, a repeater station which uses phase-conjugate light, is arranged between a transmitter station and a receiver station, and is laid through an optical transmission line is disclosed (for example, in Japanese Registered Patent No. 3436310). The repeater station includes a phase-conjugate-light generating device and a modulation unit. The phase-conjugate-light generating device has a control-light/pump-light supply unit and a control-light/phase-conjugate-light extraction unit. The control-light/pump-light supply unit supplies to a nonlinear optical medium pump light and input control light which is transmitted from the transmitter station, and the control-light/phase-conjugate-light extraction unit extracts output control light and phase-conjugate light which are generated by the input control light and pump light supplied to the nonlinear optical medium. The modulation unit modulates the pump light with monitor data unique to the repeater station. The repeater station transmits the phase-conjugate light to the receiver station, where the phase-conjugate light contains the modulated monitor data.
However, there is a problem that when carrier light on which signals are multiplexed is branched from an optical transmission line to receive the signals, an influence such as power loss is exerted on the carrier light propagating through the optical transmission line.